The invention relates to a flight control system, especially for helicopters. Such flight control systems have at least one mechanical control link for transmitting a mechanical control signal and an internal closed loop control cooperating with the mechanical control link. The internal closed loop control has two pick-ups for providing two electrical signals, one of which is picked-up at the input of the mechaical control link and the other of which is pick-up at the output of the mechanical control link. A circuit arrangement for producing a signal difference between the two pick-up signals forms part of the closed loop control. This difference signal represents a control deviation which is used for producing a correction signal. The correction signal in turn is used to control a servodrive connected to the mechanical control link just downstream of the signal pick-up at the input of the mechanical control link as viewed in the signal flow direction.
Prior art mechanical control links, especially having a plurality of individual elements as they are used, for example, in flight control systems are subject to a nonlinear hysteresis type of signal transmission. Such transmission may have a plurality of causes, including mechanical play, friction effects, elasticity phenomena and combinations thereof. As a result of these nonlinear hysteresis type characteristics of prior art mechanical control links such links are subject to inaccuracies in the control as well as to continuous oscillations having a relatively low frequency and a large amplitude resulting in so-called limit cycles. Particularly in connection with helicopters equipped with such control systems the limit cycles have been found to be rather disturbing, especially with regard to the flight characteristics of the helicopter.
Efforts to mechanically reduce such characteristics of mechanical control links have had limited success, if any, and such mechanical efforts involve a high manufacturing investment and high costs. Mechanical efforts to reduce such characteristics have their own limitations, because, for example, making the mechanical control link shorter cannot be accomplished beyond a certain point and the same applies to constructing the movable joints of the mechanical control link to have as little play as possible.
It has been found that in a control system as described above, the instabilities in the form of continuous oscillations or limit cycles could not be eliminated. At best, such instabilities may could be slightly reduced. This applies especially in those systems where automatic control signals are applied to the mechanical control link. Such automatic control signals may come, for example, from an automatic pilot in addition to, or instead of the manual control signals given by the pilot.